Nuclear magnetic resonance (NMR) or magnetic resonance imaging (MM), functional MRI (fMRI), electron spin resonance (ESR) or electron paramagnetic resonance (EPR) and other imaging techniques using RF field generating coils are finding increasing utility in applications involving imaging of various parts of the human body, of other organisms, whether living or dead, and of other materials or objects requiring imaging or spectroscopy.
Magnetic Resonance Imaging (MRI) generally utilizes hydrogen nuclear spins of the water molecules in the subject (which may be a human body, other organism, or non-organism), although other nuclear spins have been used as well. The spins are polarized by a strong, uniform, static magnetic field (conventionally denoted as B0) typically generated by a superconducting magnet. The magnetically polarized nuclear spins generate magnetic moments in the subject. The magnetic moments point or are aligned parallel to the direction of the main magnetic field B0 in a steady state and generally do not produce useful information if they are not disturbed by any excitation.
The generation of nuclear magnetic resonance (NMR) signals for MRI data acquisition may be accomplished by exciting the magnetic moments with a uniform radio-frequency (RF) magnetic field (typically referred to as the B1 field or the excitation field), for example, by applying a uniform RF magnetic field orthogonal to B0. This RF field typically is centered on the Larmor frequency of protons in the B0 field and causes the magnetic moments to mutate their alignment away from B0. The B1 field typically is produced in the imaging region of interest by an RF transmit or drive coil that is driven by a computer-controlled RF transmitter with a RF power amplifier. During excitation, the nuclear spin system absorbs magnetic energy, and the magnetic moments process around the direction of the main magnetic field. After excitation, the processing magnetic moments go through a decay process, release their absorbed energy, and return to a steady state. During the decay process, NMR signals may be detected by the use of a receive RF coil that is placed in the vicinity of the excited volume of a subject. The NMR signal is an electrical voltage or current in the receive RF coil that has been induced by the flux change over a period of time due to the relaxation of processing magnetic moments. In a conventional MRI system, imaging may be assisted by the use of additional pulsed magnetic gradient fields to result in selective excitation of specific volumes of the subject, thus spatially encoding the NMR signal to correspond to those specific volumes. These gradient field may be generated by gradient coils integrated inside the main magnet system.
A recent trend in MRI technology has been the development of sophisticated multi-element phased array coils that are capable of acquiring multiple channels of data in parallel. Such “parallel imaging” techniques may have a number of advantages, such as accelerated imaging, in some cases by replacing some of the spatial coding originating from the magnetic gradients with the spatial sensitivity of the different coil elements.
There is an ongoing need for new and improved RF coils, including coils used for parallel imaging.